4 files changed, 150 insertions, 0 deletions
diff --git a/object/signature/parse.go b/object/signature/parse.go
new file mode 100644
index 00000000..a6880eee
--- /dev/null
+++ b/object/signature/parse.go
@@ -0,0 +1,97 @@
+package signature
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"strconv"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+)
+
+// Parse parses a canonical Git signature line:
+// "Name <email> 123456789 +0000".
+func Parse(line []byte) (*Signature, error) {
+	lt := bytes.IndexByte(line, '<')
+	if lt < 0 {
+		return nil, errors.New("object: signature: missing opening <")
+	}
+
+	gtRel := bytes.IndexByte(line[lt+1:], '>')
+	if gtRel < 0 {
+		return nil, errors.New("object: signature: missing closing >")
+	}
+
+	gt := lt + 1 + gtRel
+
+	nameBytes := append([]byte(nil), bytes.TrimRight(line[:lt], " ")...)
+	emailBytes := append([]byte(nil), line[lt+1:gt]...)
+
+	rest := line[gt+1:]
+	if len(rest) == 0 || rest[0] != ' ' {
+		return nil, errors.New("object: signature: missing timestamp separator")
+	}
+
+	rest = rest[1:]
+
+	before, after, ok := bytes.Cut(rest, []byte{' '})
+	if !ok {
+		return nil, errors.New("object: signature: missing timezone separator")
+	}
+
+	when, err := strconv.ParseInt(string(before), 10, 64)
+	if err != nil {
+		return nil, fmt.Errorf("object: signature: invalid timestamp: %w", err)
+	}
+
+	tz := after
+	if len(tz) < 5 {
+		return nil, errors.New("object: signature: invalid timezone encoding")
+	}
+
+	sign := 1
+
+	switch tz[0] {
+	case '-':
+		sign = -1
+	case '+':
+	default:
+		return nil, errors.New("object: signature: invalid timezone sign")
+	}
+
+	hh, err := strconv.Atoi(string(tz[1:3]))
+	if err != nil {
+		return nil, fmt.Errorf("object: signature: invalid timezone hours: %w", err)
+	}
+
+	mm, err := strconv.Atoi(string(tz[3:5]))
+	if err != nil {
+		return nil, fmt.Errorf("object: signature: invalid timezone minutes: %w", err)
+	}
+
+	if hh < 0 || hh > 23 {
+		return nil, errors.New("object: signature: invalid timezone hours range")
+	}
+
+	if mm < 0 || mm > 59 {
+		return nil, errors.New("object: signature: invalid timezone minutes range")
+	}
+
+	total := int64(hh)*60 + int64(mm)
+
+	offset, err := intconv.Int64ToInt32(total)
+	if err != nil {
+		return nil, errors.New("object: signature: timezone overflow")
+	}
+
+	if sign < 0 {
+		offset = -offset
+	}
+
+	return &Signature{
+		Name:          nameBytes,
+		Email:         emailBytes,
+		WhenUnix:      when,
+		OffsetMinutes: offset,
+	}, nil
+}
diff --git a/object/signature/serialize.go b/object/signature/serialize.go
new file mode 100644
index 00000000..3f60d20d
--- /dev/null
+++ b/object/signature/serialize.go
@@ -0,0 +1,33 @@
+package signature
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// Serialize renders the signature in canonical Git format.
+func (signature Signature) Serialize() ([]byte, error) {
+	var b strings.Builder
+	b.Grow(len(signature.Name) + len(signature.Email) + 32)
+	b.Write(signature.Name)
+	b.WriteString(" <")
+	b.Write(signature.Email)
+	b.WriteString("> ")
+	b.WriteString(strconv.FormatInt(signature.WhenUnix, 10))
+	b.WriteByte(' ')
+
+	offset := signature.OffsetMinutes
+
+	sign := '+'
+	if offset < 0 {
+		sign = '-'
+		offset = -offset
+	}
+
+	hh := offset / 60
+	mm := offset % 60
+	fmt.Fprintf(&b, "%c%02d%02d", sign, hh, mm)
+
+	return []byte(b.String()), nil
+}
diff --git a/object/signature/signature.go b/object/signature/signature.go
new file mode 100644
index 00000000..22e516f9
--- /dev/null
+++ b/object/signature/signature.go
@@ -0,0 +1,10 @@
+// Package signature provides routines and representations that implement author/committer/tagger signatures.
+package signature
+
+// Signature represents a Git signature (author/committer/tagger).
+type Signature struct {
+	Name          []byte
+	Email         []byte
+	WhenUnix      int64
+	OffsetMinutes int32
+}
diff --git a/object/signature/when.go b/object/signature/when.go
new file mode 100644
index 00000000..0a252f68
--- /dev/null
+++ b/object/signature/when.go
@@ -0,0 +1,10 @@
+package signature
+
+import "time"
+
+// When returns a time.Time with the signature's timezone offset.
+func (signature Signature) When() time.Time {
+	loc := time.FixedZone("git", int(signature.OffsetMinutes)*60)
+
+	return time.Unix(signature.WhenUnix, 0).In(loc)
+}